Chromium alloy steel manufacture



Oct. 17, 1939. M 1 uw I 2,116,686

CHROMIUM ALLOY STEEL MANUFACTURE n Filed Jan. 22, 1938 sucia as nl fate ATTORNEYS iatented Oct. i7, i939 UNITED STATES PATENT @Fl-TICE CHROMIUM ALLOY .STEEL MANUFACTURE Marvin J. Udy, Niagara Falls, N. Y. Application January 22, 1938, Serial No. 186,465 i8 Claims. (Cl. 'l5-27) This invention or discovery relates to chromium alloy steel manufacture; and it comprises as a new article of manufacture useful in making low carbon chromium-iron alloys a solid mass, advantageously in the form of briquettes, or blocks, capable of undergoing internal reactions with production of a molten body of such an alloy, said mass being composed of an intimate admixture of a linely ground iron oxide, fine grained ierrochrome silicon low in carbon,high in silicon and in proportion to reduce the iron oxide and ilnely divided lime in amount to convert the silica to be formed in the reduction into a basic lime silicate slag; and it further comprises processes of utilizing chromite ores for making low-carbon chromium-iron and chromium-steel alloys wherein a ferrous chromite lore of any grade is reduced with carbon to high carbon :ferrochromium with elimination of the ore gangue, the ferrochromium is re-smelted with silica and carbon to a highly siliconized ferrochrome metal low in carbon and this metal, carrying most of the chromium and iron of the original ore, is mixed in finely divided form with iron oxide and lime, and the silicon of the metal is removed therefrom and replaced with iron by exothermic reductlonof the iron oxide forming the low carbon chromium-iron alloy and a basic slag, both in molten form and readily separable; all as more fully hereinafter set forth and as claimed.

In nearlyk all the commercial chromium-iron alloys such as stainless steel, rustless iron, etc., a low carbon content is required. The chromium content may vary from a fraction of one per cent up to 30 per cent or over. Other alloying metals are usually present. In nearly all cases these alloys have been produced by the use of commercial ferrochromium having chromium in a ratio to iron of around 70:30. The reason for using these high chromium ferro alloys is-that they are correspondingly lc-w in the ratio of carbon to chromium. They can be produced only from the highest grade chromite ores low in iron content. It is an economic waste to use high grade ores to produce a rich ferrochromium and then dilute this metal With more iron to form commercial alloys in which chromium is only a minor constituent.

It is anv object achieved in the present invention to provide a material and method for making the low carbon finished steel or iron alloyed with chromium and other metals in minor lproportions without dependence upon high grade chromite ores of low iron ratios. It is no longer necessary to high grade the natural chromite ore deposits of the world and to leave unutilized plentiful supplies oi' rich ferrous chromite mineral because of its low chromium to iron ratio.

I have discovered that iron-chromium alloys of predetermined chromium and carbon contents 5 are made with great facility by exothermic reaction, in nely divided admixture, of iron oxide with ierrochromium siliconized to. low carbon and high silicon contents sufficient to reduce the iron oxide and with an amount of lime to convert l0 the silica formed in the reduction to a silicate slag containing lime in `more than an equimolecular ratio to the silica. 'I'he iron oxide in being reduced to iron by silicon dilutes the chromium and the reaction heat of the finely l5 divided admixture is in excess of the amount required to melt the silicate; both the dilution and the excess heat, as Well as the carbon content of the metal product, being predetermined by the percentage of silicon in the ferrochrome-silicon, 20 that is by the degree of siliconization of the ferrochromium in adxnixture with iron oxide. As I have found, the silicon in the ferrochromesilicon component with admixed lime in more than a 2:1 `proportion by weight to the silicon g5 protects the chromium against oxidation by the iron Without requiring silicon in great excess. The lime in the mixture combining with silica promotes the oxidation of silicon by iron oxide and adds to the heatI of the internal reaction 30 taking place in the mixture when ignited.

A Silico-thermic briquetted mixture of ferrochrome silicon, iron oxide and lime, in proportions giving on reaction a basic silicate slag and a chromium-iron metal containing residual 35 silicon in amounts of the order of l per cent of the metal or less and a low carbon. content resulting from dilution of the low-carbon ferrochrome metal by the exothermicallyr` reduced iron, is a highly advantageous material for making any oxidation-resistant high strength alloy steel or iron. One of its outstanding advantages is that it can be made from chrome iron ores of any chromium-iron ratio, even from an iron ore containing minor amounts of chromium, for example, a Cuban iron ore of 2-4 per cent chrolmium content ora 1:20 chromium-iron ratio.

Another advantage is that there can be incorporated in the Silico-thermic mixture other alloying constituents such as nickel, molybdenum, 5o manganese, vanadium, tungsten, etc. Wanted in the iinished metal. These may be incorporated either asV oxides in the iron oxide component or as metals in the ferrochrome silicon component. In the latter case silicate ores of the metals can be utilized as sources of silicon for the siliconized metal. These other alloying metals may, however. be added to the molten chromium-iron metal formed upon ignition .of the mixture and exothermic reaction therein. The metal product of reaction of the three-component mixture may be diluted further with molten steel or iron to adjust the chromium content as desired.

It is sometimes advantageous to incorporate in the mixture mme saline oxidant as sodium chlorate, or nitrate or chromate, with excess ierrochrome silicon over that needed to reduce the iron oxide. This gives heat in oxidizing silicon without diluting the ferrochrome metal with iron. It increases the amount of heat available for melting the slag and the metal. The exothermicity of the mixture may thus .be increased. It is usually better to have the proportion of iron oxide and saline oxidant to the silicon a little less than that suillcient to oxidize all the silicon. A little silicon in the metal in the presence of lime protects the chromium from oxidation. Chromate used as oxidant adds chromium to the metal.

I have found it highly advantageous to add to the exothermic mixture a small proportion of chromated ferrichromite made by roasting high carbon chromium or ferrochromium.

Not the least of the 'advantages of the silicothermic material for making alloy steel is that it can be directly converted into such an alloy by being introduced into an open hearth steel furnace hot enough to start the exothermic reaction within the material. An ordinary steel making temperature around 1500 to 1600 C. is advantageous and the material, which may carry within itself all or the greater part of the heat energy required to melt the reaction products to a free running metal and slag, may be slowly fed to the furnace. Thus working, any deiiciency of heat required to finish the alloy, vorto add metallic alloying constituents in addition to the chromium, may be supplied by the usual means for heating the furnace. This allows some latitude in the silicon content of thesiliconised ferrochromium, which in the last analysis deter- -comitantly the proportion of other useful constituents.

High carbon ,ferrochromium made from chromiteoreofanygradebywellknownsmelting methods is readily siliconid to any desired extent up to,say.75percentbysmeltinwithsilica' and carbon as coke. Pen'ochromem thus madeandcontoiningopereentsiliconormore isextremelylowin'earboncmtent. 'niismetal inadmixture with iron oxideand lime,inpro portions as described, is converted by exothermic actioninanopenbearthsteelfurnacetoacarbon-free metal of a diluted chromium content and a lime silicate slag; with development of a quantity of heat not far from that needed to melt the diluted metal and the slag. In the reaction. 3Si+2FezOa=4Fe+3Si0a+22300 calories. each unit of silcon in ferrochrome-sllicon being replaced by 2.66 units of iron, a 10:30 ferrochrome with 50 per cent Si, for example, is diluted to 18 pound oi silicon; the silcon being converted with lime into 4.5 to 5 pounds lime silicate slag. A positive heat development in the silicate formation of 900 cal. per unit Si, added to 2600 cal. for the reduction brings the exothermic heat to 3500 centigrade units per pound Si. This amount of heat balances 1500 heat units used in melting 3.68 pounds metal and 2000 units in 4.5 to 5 pounds slag. The production of carbon-free 18 per cent chromium metal from a mixture of ferrochrome silicon and iron oxide with a slag ratio of 1.25 may thus be made self sustaining.

In some cases it may be advantageous to siliconize the ferrochrome component to a moderate silicon content of 30 to 40 per cent in order to increase the proportion of chromium or other alloying metal in the silicon-thermic material. This may avoid undue dilution of the chromium by iron in the finished alloy. But it is usually better to run the silicon up to 50 per cent or more of the ferrochrome silcon so as to have more latent available heat to add to the heat of the steel furnace. By adding sodium chlorate or chromate or nitrate silicon supplies heat Without diluting the metal with iron formed in reduction of iron oxide.

It may be noted that the higher the percentage of silicon in the ferrochrome silicon, the lower is the carbon content and the greater is the dilution of the ferrochrome and the carbon content with iron replacing the silicon by virtue of the reduction of the oxide component in the silicothermic mixture. Thus high silicon in the ferrochrome component conduces in two ways to low carbon in the alloy steel made by igniting themixture.

As to the silicon content of the ferrochrome silicon, it is found'that a given silicon content insures a lower carbon content, the lower the ratio of chromium to iron in the ferrochromium which is siliconized. For example, in a ferrochromium of a Cr-Fe ratio 1:1 or less, 40 per cent Si insures a carbon content under 0.1 per cent. With extremely low chromium contents, under 5 per cent, carbon is below 0.10 per cent with to per cent Si in the metal. 'Ihus there is particular advantage in utilizing substandard. chromite ores for making the ferrochromium to be siliconized. For example, a 40 per cent silicon metal of a 1:1 Cr/Fe ratio can be converted to an 18 per cent chrome steel of a carbon content under 0.10 per cent by exothermic reaction with lime. iron oxide and sodium chlorate or nitrate in proportions giving a reaction heat suiiicient to melt the metal and silicate slag. If a somewhat larger chromium content in the steel is required, it may be obtained by putting a somewhat smaller silicon content in the ferrochrome silicon to be replaced by iron. If a smaller chromium content is wanted in the chrome steel, it is a simple matter to dilute with iron the chromeiron alloy formed by reaction of the silico-thermic mixture. Likewise, the chromium content of the iinished alloy is raised or lowered by incorporating more or less saline oxidant in the silicothermic material.

It is advantageous, as I have found, to produce the ferrochrome silicon by siliconizlng high carbon ierrochromium made by smelting a chromite ore with carbon in a well known way. In this smelting the process described in my copending application Serial No. 165.954, filed September 27.

r 1937. (continued in Serial No. 228,861, filed Sept. 7, 1938) gives a high yield of chromium from the u per cent chromium metal, or 3.68 pounds per ore by putting a small amount of silicon into the Il metal. This metalcan then 'be smelted with and carbon to produce siliconized ferrochromium ofl a high silicon content which ensures a carbon 4content of less than 0.1 per cent. -Asabove stated,

the low grade or high iron ferrochromiums lend themselves particularly well to lowering thecarbon content by addition of silicon. In this operation it may be advantageous to use as-source of silicon a nickel silicate ore in order to putinto the siliconized metal a desired content of nickel `with the chromium. A silicon content of 50 per cent or over is advantageous not only in lowering the carbon content, but also in providing in the silico-thermic mixture a good supply of latent exothermic heat per unit weight of the material.

For the iron oxide to be mixed with the siliconized ferrochromium, mill scale or the magnetic oxide may. serve. This requires less heat for reduction per unit of iron than the ferrie oxide.

' But the magnetic oxide/dilutes the ferrochrome metal to a greaterfextent than ferrie oxide or hematite. Either oi the oxides is available for my purposes; the purer the oxide the better. Oxide orescontaining small amounts oi oxides of metals desired as minor alloying constituents of the chrome steel alloy may be used to advantage;

for example, iron ores containing vanadium' oxides.

In a speciiic embodiment of theinv'ention I use as source of chromium in the exothermic.`

mixture a low grade chromite ore .0f the follow- 'Ihis ore is reduced in a submerged arc electric furnace with carbon in the presence of lime and silica in .proportions giving a normal dibasic silicate slag and an excess of silica for reduction, as described in my mentioned copending application Serial No. 165,954. The lreduction results in a high yield of metal having the following composition:

" iinely ground iron o re containing 60 per cent iron,

Parts Ferrochrome silicon, 40 per cent Si 150 Iron ore, 60 per cent Fe 210 Nickel oxide (N10) 23.9 Lime.- CaO 192 l NaClOs- 21.2

a. small amount of pure nickel oxide, lime and a v little sodium chlorate as an oxidant for the silicon. The proportions are:

These materials are all together' in avball v'rhesiett amounts to 32o pounds, which mill to pass mesh. This mixture is then brie quetted, or it can be packed in cans. For ignition of the mixture a temperature of 800 to 900 C. sumces. To make the alloy metal the briquetted mixture is `fed to an open hearth furnace heated to 1500 C. The reaction proceeds smoothly and rapidly and there is produced 225 pounds molten steelanalyzing as follows:

t Per cent Chromium 17.3 Nielsen. 8.3

vCarboni 0.05 Silicon 0.4

is in a ratio to the steel of 1.4 to 1.

It is noted that in the above example the saline oxidant sodium chlorate is used to leave only a. little excess silicon over that required in being replaced by iron to dilute the metal to the desired composition. This is advantageous in increasing the exothermicity of the mixture and it maybe utilized to give a fully self-sustaining reaction. It is further noted that by lowering the silicon in the ferrochromium to 35 per cent the reaction with iron oxide `gives the same metal -composition without the use of chlorate. With the lowered silicon, the slag volume would be less. It is noted that the silicon is used entirely to reduceI oxides other than chromium oxide.

In another specic embodiment of my invention I-utili'ze a Puerto Rican limonite iron ore fwhich on roasting analyzes as follows:

' bon and 4 per cent silicon.

Per cent FezOs, Fe 49 per cent '10 A1203-- 23.0 S102 2.7

N10, Ni 0.78 per cent 1.0 CraOs, Cr 1.3 per cent 1.9

I next smelt this metal with silica and coke tol produce 76 parts of a siliconized metal containing 1.58 per cent chromium and 0.92 per cent nickel with'30 per cent silicon and carbon under 0.10 per cent. This melting .is readilydone in an electric furnace ofthe submerged arc type. The same smelting can be accomplished in a blast furnace. 1

The 'lparts of siliconized metal made from 100 parts of the original limonite ore are admixed with parts of ,the same ore and 70 parts o! lime. preferably in the form of briquettes; the three components being ground to 100 mesh. The briquettes are fed to 'a bath of 280 parts molten iron refined to thhdesired carbon content. The metal formed in added to the iron produces 400 parts oi steel containing 0.5 per centchromium and 0.44 per cent nickel with less than 0.10 per cent carbon. The slag is lime silicate containing about 1.4 parts eso w 1 alos. The retto ef sleg tot the steel predueed is about o.3; 1. It 1s. noted that it is not necessary to remove carbon from low chromium e briquette internal reaction iron when a-low chromium steel is produced.

Carbon adjustments are readilymade togive the silicon from the siliconized metal entirely with iron ore, I may add to the iron ore a desired ratio of calcium ferrichromite-chromate of any required high chromium content, as described in.

my accompanying application Serial No. 186,466. An artificial chromite i'or this purpose is disclosed in my copending application Serial No. 165,417, led September 23, 1937. The artiilcial chromite or calcium ferri-chromite-chromate may be made to great advantage by roasting high carbon ferrochromium of any Cr/Fe ratio in nely divided admixture with lime and a little soda ash at about 1000 C. The roasted product is composed of CaO, FezOa, CrzOa and CrO: in chemical combination, the lime being most advantageously in proportion to flux the silica formed in exothermic reduction of the chromium and iron oxides by silicon. It is noted that a greater content of silicon than per cent may be Dut into the ferrochrome silicon metal to supply a greater amount of heat to the steel furnace in the exothermic mixture. Instead of adding to this silicon content, it may be advantageousto use ferroscilicon or silicon in admixture with ferrochrome silicon and ironore.

In the accompanying drawing, I have shown for purposes of illustration only, and not for purposes of limitation, a flow sheet illustrating several of the possible chromium recovery processes which may be carried out in employing the principles of the invention. Heavy lines have been employed to outline a complete process commencing with the treatment of ore containing chromium and iron and indicating the production ultimately of desirable chromium-bearing metal products. In dotted lines, there is indicated a modified process employing'oxidized ferrochromium, and, in light lines, I have indicated the use of alloying elements such as nickel, molybdenum, manganese, vanadium and tungsten.

What I claim is:

1. As a composition useful in making alloy steels containing chromium, an intimate mixture of iron oxide, ferrochrome silicon low in carbon and containing suilicient silicon to reduce all the iron oxide to metal and lime suiiicient to ux the silica formed in said reduction. said composition being capable of converting itself by internal exothermic reactions into a ferrous chromium alloy and a basic lime silicate slag.

2. A material for making chromium alloy steel composed of iron oxide, low carbon ferrochrome silicon containing silicon suilicient to reduce the iron oxide to metal and lime sumcient to flux the silica formed in said reduction to a basic silicate slag, all in finely divided admixture capable of converting itself by internal exothermic reaction into chromium alloy steel and lime silicate slag.

3. A material according to claim 2 in which the proportions of the three components in finely divided admixture are such as to form by exothermic reaction molten alloy steel and molten slag.

4. A silico-thermic material comprising a mixture of, iron oxide, lime and sillconized ferro-` chromium of controlled carbon content in iinely divided condition and in proportions to form by internal exothermic reaction lime silicate slag having more than an equimolecular ratio or lime to silica and a chromium-iron metal oi' predetermined carbon and silicon contents.

5. A silice-thermic material comprising a mix-A ture oi iron oxide, lime, siiiconized ierrochromium ofcontrolled carbon content and a saline oxidant in iinely divided condition and in proportions to form by internal exothermic reaction low-carbon ferrous metal alloyed with chromium and lime silicate slag having more than an equimolecular ratio of lime to silica.

6. A silico-thermic material composed of iron oxide, nickel oxide, siliconized ferrochromium, a saline oxidant and lime in finely divided intimate admixture in relative proportions such as to form by internal exothermic reactions low silicon 1errous metal alloyed with chromium and with nickel and lime silicate lslag having a CaOzSiOz molecular ratio greater than 1:1.

7. A process of making chromium alloy steel or iron from chromite ores which comprises reducing a chromite or'e with carbon at a smelting temperature to high carbon ferrochromium .with iluxing `elimination of the ore gangue, siliconizing and removing carbon from said ierrochromium by smelting with silica and carbon, grinding said siliconized metal and mixing it with ilnely divided iron oxide and lime and igniting the mixture.

8. In utilizing natural ferrous chromite ores for making chromium alloy steels, a process which comprises reducing such an ore of any grade with carbon to high' carbon ferrochromium, smelting said ferrochromium with silica and carbon to ferrochrome silicon high in silicon and low in carbon, admixing said ferrochrome silicon in finely divided form with finely divided lime and finely divided iron oxide in relative quantities such as to convert most o1' the silicon to calcium silicate and igniting said admixture to react exothermically and form the nished chromium alloy with calcium silicate slag.

9. In making chromium-iron alloys, a method of utilizing chromite ores which comprises making from chromite ore of any grade ferrochromesilicon metal of high silicon content insuring a low carbon content, grinding said metal and mixing it with finely divided iron oxide and lime and igniting said mixture to form alow carbon chromium-iron metal by exothermic reaction.

10. In making chromium alloy steel or iron, a method of utilizing chromite ores which comprises making from chromite ore of any grade ferrochrome silicon metal of high silicon content insuring a low carbon content, grinding said metal and mixing it with finely divided iron oxide, a saline oxidant and lime in quantities suiiicient to oxidize said silicon and igniting said mixture to form a chromium-iron metal by exothermic reaction.

11. A method of making low carbon chromium steel from low grade chromite ores of high iron content which comprises making from such an ore having a chromium-iron ratio less than 2:1 a ferrochrome-silicon metal of high silicon content insuring a low carbon content, grinding said metal and mixing it with iinely divided iron oxide and lime and igniting said mixture to form a chromium-iron metal by exothermic reaction with formation of a lime-silicate slag.

12. A silica-thermic material composed of ferro-chromeisilicon metal having a Cr/Fe ratio less than 2:1,.iron oxide and lime in an intimate mixture of particle sizes pass'ng a screen of 100 meshes to the linear inch and capable of converting itself by internal exothermic reactions into chromium alloy steel and lime-silicate slag.

13. A method oi' making a silico-thermic material for use in producing chromium-iron' alloys which comprises, fine-grinding a ferrochrome silicon metal of highsilicon and correspondingly low carbon content and intimately mixing the ground metal with iinely ground ironjoxide andlime in about the quantities required to oxidize the silicon of the metal with formation of a lime-silicate slag containing lime in at least an equimolecular ratio to the silica.

14. A material for use in making alloy steel by exotherinic action which comprises an intimate mixture of finely divided iron oxide, ferrochrome silicon and burned lime, the lime being in a ratio to the silicon of the ferrosilicon somewhat greater than 2 parts by weight of lime to one part of silicon.

15. A silico-thermic material capable of converting itself by internal exothermic reaction into* nickel-chromium alloy steel, said material beingy an intimate mixture of iron oxide, a ferrochrome silicon metal containing nickel and lime in proportions to oxidize the silicon to a lime-silicate slag having a CaO-SiOz molecular ratio above 1:1.

16. A method of producing a composition for use in making alloy steels containing chromium which comprises grinding to a flneness at least suflicient to pass a screen of meshes to the inch a mixture of iron oxide, ferrochrome silicon low in carbon and containing suiilcient silicon to reduce all the iron oxide tovmetal and lime sufficient to iiux the silica formed in said reduction to a lime silicate slag containing lime l consisting essentially of iron oxide, a saline oxidant, ferrochrome silicon containing silicon in amount suflicient to reduce the iron oxide to metal and to be oxidized by the saline oxidant and lime suillcient to iux the silica formed by oxidation of said silicon, said composition being capable of converting itself by internal exothermic reactions into a. ferrous chromium alloy and a basic lime silicate slag.

18. A silico-thermic material comprising a mixture of oxidized ferrochromium, iron oxide, lime and ferrochrome silicon of controlled cerbon content in nely dividedcondition and in proportion to form by internal exothermic reaction lime silicate slag having more than an equimolecular ratio of lime to silica and a chromiumiron metal of predetermined carbon and silicon contents.

MARVIN J. UDY. f 25 

